This invention relates to a novel fluoroolefin base multicomponent copolymer which is soluble in various organic solvents, curable at relatively low temperatures and useful as a paint vehicle.
Fluororesins are generally excellent in weather resistance, heat resistance and chemical resistance. Accordingly use of fluororesins as paint vehicles has been developed in the chemical industry, construction industry, machinery industry and food industry. Early developed fluororesin paints were mostly powder paints and emulsion paints because of poor solubilities of readily available fluororesins, and these paints required a high temperature baking treatment. Recently much attention has been directed to fluororesin paints using a fluorine-containing copolymers which is soluble in organic solvents suited to paints and is curable at normal temperature. Fluororesin paints of this type are comparable to ordinary synthetic resin paints in the ease of application and provide paint films superior in chemical resistance and weather resistance.
To modify a fluororesin into another fluororesin which is soluble in practicable organic solvents, usually it is necessary to reduce crystallinity of the fluororesin by copolymerzation to thereby accomplish internal plasticization. Furthermore, there are some other problems to be resolved in using the modified fluororesin as a paint vehicle. The problems include how to retain a desirable degree of rigidity or shear modulus in the modified fluororesin, how to control the molecular weight of the modified resin with a view to desirably adjusting the viscosities of paints and how to select and control the kind and amount of functional groups which are introduced into the modified resin in order to render the resin curable and recoatable. Cost of production also has to be taken into consideration. It is not easy to reach balanced solution for all problems.
U.S. Pat. No. 4,631,326 shows a copolymer of chlorotrifluoroethylene, a vinyl or isopropenyl ester of fatty acid and a hydroxyl-containing allyl ether. This copolymer is soluble in various organic solvents and curable at normal temperature by using a polyisocyanate as curing agent. Therefore, a solution of this copolymer is useful as a liquid vehicle of a paint or coating composition. However, solutions of this copolymer are not satisfactory in respect of dispersibility of organic pigments in the solutions.
In using a solution of a fluororesin as a paint vehicle a matter of important concern is dispersibility of pigments in the solution. If the paint contains a pigment which is not well dispersed or is not stable in the dispersed state, the paint will exhibit unfavorable properties such as unevenness of the color of a paint film and color separation within the paint in storage.
It is known to introduce carboxyl group into a polymer as a measure for improving dispersibility of powders in solutions of the polymer. The application of this technique to fluororesins has been tried, but a relatively simple way such as the use of methacrylic acid as a hydroxyl-containing comonomer has encountered a problem that both the degree of polymerization and the yield of the polymer decrease. To obviate this problem, JP-A 58-136605 proposes first preparing a copolymer of a fluoroolefin and a hydroxyl-containing monomer such as an alkylvinyl ether and then reacting the copolymer with a dibasic acid anhydride to thereby introduce carboxyl group into the copolymer at the positions of at least a portion of the hydroxyl groups. From a practical point of view, this method has disadvantages such as the necessity of performing two reactions, a relatively high temperature (about 100.degree. C.) for the second reaction in which one of the reactants is a polymer and a tendency to coloring of the modified copolymer.